We propose to study the mechanism of recombination and of the synthesis of viral DNA during the first cycle of avian sarcoma viruses (ASV) infections. Using molecular cloning, electronmicroscopy, transfection, molecular hybridizations with highly defined reagents, fragmentation of DNA with restriction endonucleases, the DNA transfer technique of Southern and the DNA sequencing technique of Maxam and Gilbert, we wish to characterize (1) the stage of viral replication cycle at which recombination occurs; (2) the structural features of the possible recombinational intermediates, which include heterozygous genomes, linear DNA heterodimers, and circular DNA heterodimers; (3) the mode of synthesis of viral second strand DNA, including the nature of the primer molecules, the initiation sites, the sequence specificity of the initiation process; (4) the factors involved in viral second strand DNA synthesis; (5) the mechanism by which linear DNA circularizes and (6) by DNA sequencing the portions of linear and circular viral DNAs involved in integration event.